1. Field of the Invention
The present invention relates to an X-ray imaging device that obtains an X-ray image of a subject, by detecting X-rays that have passed through the subject.
2. Description of the Related Art
Presently, various X-ray imaging apparatuses, for obtaining X-ray images to be utilized for medical diagnoses, have been proposed and are in practical use. These X-ray imaging apparatuses employ solid state detectors (having semiconductors as main components thereof) as X-ray image detecting means. The solid state detectors detect X-rays that have passed through subjects, and obtain image signals that represent X-ray images of the subject.
A variety of formats have been proposed for the solid state detectors to be utilized in these apparatuses. Regarding a charge generating process for converting X-rays to electrical charges, there is a photo conversion type of solid state detector, and a direct conversion type of solid state detector, for example. The photo conversion type of solid state detector temporarily stores signal charges, obtained at a photoconductive layer by detecting fluorescence emitted by phosphors due to irradiation with X-rays, in a charge accumulating portion, then converts the accumulated charges to image signals (electrical signals) and outputs the image signals. The direct conversion type of solid state detector temporarily stores signal charges, generated within a photoconductive layer due to irradiation with X-rays and collected by a charge collecting electrode, in a charge accumulating portion, then converts the accumulated charges to electric signals and outputs the electric signals. In this type of solid state detector, the main components are the photoconductive layer and the charge collecting electrode.
Regarding a charge readout process for reading out the accumulated charges, there are an optical readout technique and a TFT readout technique. In the optical readout technique, accumulated charges are read out by irradiating a solid state detector with readout light (electromagnetic waves for readout). In the TFT readout technique, accumulated charges are read out by scanning TFT's (thin film transistors), which are connected to a charge accumulating portion. The TFT readout technique is disclosed in Japanese Unexamined Patent Publication No. 2000-244824.
An improved direct conversion type solid state detector has also been proposed in U.S. Pat. No. 6,268,614. The improved direct conversion type solid state detector is a direct conversion type of solid state detector that utilizes the optical readout technique. This solid state detector comprises: a recording photoconductive layer that exhibits photoconductivity when irradiated by recording light (X-rays, or fluorescence generated by the irradiation of X-rays); a charge transport layer that acts substantially as an insulator with respect to charges having the same polarity as latent image charges, and that acts substantially as a conductor with respect to charges having the opposite polarity as latent image charges; and a readout photoconductive layer that exhibits photoconductivity when irradiated by electromagnetic waves for readout; stacked in this order. Signal charges (latent image charges) that bear image information are accumulated at an interface (charge accumulating portion) between the recording photoconductive layer and the charge transport layer. Electrodes (a first conductive layer and a second conductive layer) are provided on both sides of the three aforementioned layers. In the solid state detector having this format, the recording photoconductive layer, the charge transport layer, and the readout photoconductive layer are the main components.
In medical X-ray imaging, there are apparatuses other than those that employ the aforementioned solid state detectors. Examples of alternative X-ray image detecting means are imaging plates and film. However, in all cases, it is necessary to control the dosage of X-rays irradiated onto the X-ray image detecting means during X-ray imaging, to obtain optimal images. Therefore, apparatuses that detect the X-ray dosage irradiated on X-ray image detecting means, and apply the detected X-ray dosage to control the irradiated X-ray dosage are disclosed in Japanese Unexamined Patent Publication Nos. 2001-305232, 58(1983)-223145, and 60(1985)-198097.
In the X-ray imaging apparatus disclosed in Japanese Unexamined Patent Publication No. 2001-305232, a solid state detector is employed as the X-ray image detecting means, and an X-ray dosage detector that detects X-ray dosages is provided on the rear surface of the solid state detector. However, if the X-ray dosage detector is provided on the rear side of the solid state detector, the accuracy in detection of the X-ray dosage is low. This is due to the fact that a large portion of X-rays is absorbed by the solid state detector. In addition, the X-rays irradiated by a radiation source possess a wide energy spectrum. However, low energy components are largely absorbed by the solid state detector, thereby changing the spectrum balance of the X-rays detected by the X-ray dosage detector.
In the apparatus disclosed in Japanese Unexamined Patent Publication No. 58(1983)-223145, an imaging plate is employed as the X-ray image detecting means, and an X-ray dosage detector is provided on the front surface of the imaging plate. However, there is no description regarding what manner of X-ray dosage detector is employed.
In the apparatus disclosed in Japanese Unexamined Patent Publication No. 60(1985)-198097, film is employed as the X-ray image detecting means, and an X-ray dosage detector is provided on the front surface of a film cassette that houses the film therein. However, the X-ray dosage detector is constituted by a fluorescent plate, a light collecting field, a light transmitting plate, and a light shielding plate, causing the structure to be complex, and limiting the degree to which the detector can be made thin. Therefore, the distance between a subject and the film for detecting an X-ray image of the subject cannot be made short, thereby decreasing the resolution of the X-ray image.
That is, no X-ray imaging apparatus has been disclosed to date that employs a solid state detector as the X-ray image detecting means and accurately detects the dosage of irradiated X-rays, to be applied toward controlling the irradiated dosage of X-rays, without adversely affecting detection of an image by the solid state detector.